AAMC Sample Test C/P #3, #46

[Takumi]

Can somebody explain the following 2 questions from AAMC Sample Test C/P Sections?


# 3 Why do kidney stones form in some individuals treated with Compound 1?

Answer: [Ca 2+] [C2O4 2-] > Ksp
Because calcium oxalate precipitates when [Ca 2+] [C2O4 2-] exceeds the solubility product constant Ksp by definition.

Shouldn't the equation goes like this: Ksp = [Ca 2+] [C2O4 2-] and the higher the [Ca 2+] [C2O4 2-] value, the higher solubility, and the less kidney stones will form? What does it mean when Ksp is smaller than the [Ca 2+] [C2O4 2-] concentration, and why does that indicates a precipitation (the forming of CaC2O4 here)?


# 46 The density of a human body can be calculated from its weight in air, W air, and its weight while submersed in water, W w. The density of a human body is proportional to:

Answer: W air / (W air - W w)
According to Archimedes' Principle, the ratio of the density of an object to the density of the fluid it is submersed in is equal to the ratio of the weight of the object in air to the difference of submersed weight and weight in air.

Can someone explain how you get this answer by applying Archimedes' Principle? Isn't the Archimedes' Principle states that "An object which is completely or partially submerged in a fluid experiences an upward force equal to the weight of the fluid displaced", Fb = V rho g = mg ? How do you derive to the sentence "the ratio of the density of an object to the density of the fluid it is submersed in is equal to the ratio of the weight of the object in air to the difference of submersed weight and weight in air" ?


Thank you very much!!!

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[salemstein]

43: actual [Ca 2+] [C2O4 2-] in the experiment is the Qsp, not Ksp. Ksp is ALWAYS the same at a given temperature. If Qsp>Ksp, then it will precip.

I had trouble with 46 as well

 

[aldol16]

Shouldn't the equation goes like this: Ksp = [Ca 2+] [C2O4 2-] and the higher the [Ca 2+] [C2O4 2-] value, the higher solubility, and the less kidney stones will form? What does it mean when Ksp is smaller than the [Ca 2+] [C2O4 2-] concentration, and why does that indicates a precipitation (the forming of CaC2O4 here)?

You're confusing a reaction quotient with solubility product constant. The solubility product constant, Ksp, is defined such that Ksp is the reaction quotient at which the solid crashes out. The reaction quotient is [Ca 2+]*[C2O4 2-]. Therefore, whenever the product of [Ca 2+] and [C2O4 2-] exceeds the Ksp, your compound crashes out. That's all it's saying.

46 is on here somewhere too - you can probably find it by searching for it. It's a bit involved and I'm pretty tired now.

 

[Takumi]

43: actual [Ca 2+] [C2O4 2-] in the experiment is the Qsp, not Ksp. Ksp is ALWAYS the same at a given temperature. If Qsp>Ksp, then it will precip.

I had trouble with 46 as well

Thank you so much!!!

Now I remembered that Ksp won't change depends on ions concentration...Do you mean that when the [Ca 2+] [C2O4 2-] concentration is bigger than the constant Ksp, that means the solution is saturated and they start to precipitate out?

 

[Takumi]

You're confusing a reaction quotient with solubility product constant. The solubility product constant, Ksp, is defined such that Ksp is the reaction quotient at which the solid crashes out. The reaction quotient is [Ca 2+]*[C2O4 2-]. Therefore, whenever the product of [Ca 2+] and [C2O4 2-] exceeds the Ksp, your compound crashes out. That's all it's saying.

46 is on here somewhere too - you can probably find it by searching for it. It's a bit involved and I'm pretty tired now.

Thank you Thank you! Make sure to take study breaks, haha.